The present invention relates to a method for obtaining environmentally durable honeycomb core structures and the product produced thereby. More particularly, this invention relates to a method for preparing a honeycomb core structure in which all surfaces are protected by the presence of an extremely adherent protective coating rendering the honeycomb core structure substantially less susceptible to corrosion, delamination and other environmentally and stress-induced failures.
A variety of methods are employed in the prior art to obtain aluminum and other metal honeycomb core structures. Heretofore, all such methods either require that the surface of the aluminum foil utilized in the manufacture of the honeycomb core be bare and thereby susceptible to corrosion or that the core material, after bonding and expansion into the well-known hexagonal cell shaped core structure, be coated with a primer material such as by dipping or otherwise pouring the primer material into the interstices of the core. In the process in which primer is inserted into the core after the core is completed, a substantial amount of the primer is utilized in forming a meniscus at each node bond in the structure. For this reason an excessive amount of primer resin is used, which adds excessive cost and weight to the structure. In addition in order to assure the complete coating of all surfaces, a relatively thick and heavy coating of the primer substance is utilized. Frequently it has been found that such primed core materials have their weight raised to an excessive and unusable level by the presence of the primer substance. Less than adequate bonding at node bonds allowing environmental and stress degradation has been found to inhibit usage of prior art honeycomb structures and is not cured by application of primer resins to the interior of the honeycomb cells.
Other structures are known in which a chemical conversion utilizing amorphous chromate coating processes is used to prepare the surface of the aluminum foil prior to bonding and expanding in to the honeycomb structure. Such processes aid in the prevention of certain types of corrosion; however, they have been shown to be unsatisfactory in situations in which the honeycomb structure is exposed for long periods of time to marine atmospheres or other highly humid conditions. In addition, inadequate bonding at the node bonds between aluminum sheets has been found even with usage of the chemical conversion treatment.
Metal honeycomb structure which comprises the interior honeycomb portion is attached to face sheets by usage of an adhesive. The resulting structure is light in weight, stiff and strong and frequently possesses good thermal and acoustical characteristics. Honeycomb core construction is widely used in aircraft construction and other transportation industries wherein a lightweight, but stiff and strong, structural component is necessary. The strength of such a composite structure relies upon the bond strength of the node bonds in the honeycomb structure itself as well as the bond strength of the core-facesheet joint. Such joints must be of sufficient strength initially to meet the strength requirements of the intended purpose and, in addition, should be resistant to degradation such as by corrosion or hydration of the oxide at the adhesive-metal interface. The strength of the bond so formed is in part dependent upon the fillet size and depth into the honeycomb cell which the fillet attains. Due to surface tension and other phenomena associated with the flow of the adhesive into the honeycomb cell, the prior art structures and methods do not form the optimum fillet size and configuration for maximum bonding strength.
It is therefore an object of this invention to provide a method of producing an environmentally stable honeycomb structure. It is a related object to produce a honeycomb structure having protection against corrosion of the metal surfaces therein to enhance the usability of such honeycomb structures in humid atmospheres.
It is a further object of this invention to provide a method of constructing honeycomb structures in which strength of the joint between the honeycomb core and the facesheets utilized is enhanced. A related further object is to provide a method for forming a honeycomb structure in which the fillet size and configuration is optimized in the joint structure between the honeycomb core and the facesheets.
It is one specific object of this invention to provide a means of manufacturing aluminum honeycomb core structures in which the aluminum sheeting material utilized in forming the honeycomb structures is coated completely with a thin coating of a corrosion-resistant resin primer material placed over an anodized, polymer-receptive oxide coating on the aluminum, and in which the further step of bonding the core materials and adhering facesheets to expanded core materials are carried out upon the primed surfaces of the aluminum honeycomb core whereby optimum adhesive fillet structure is formed to enhance the corrosion resistance and strength of the resulting honeycomb structure.
These and other objects of this invention as well as certain preferred embodiments and products will become apparent from an examination of the following description of this invention taken in conjunction with the appended drawings.